This application is a continuation of the xe2x80x9cparentxe2x80x9d European Patent Application Number 02 003 147.2, entitled xe2x80x9cTurbidity Sensor Having Adapted Transmission Characteristic And Method For Fabrication Thereof,xe2x80x9d filed with the European Patent Office on Feb. 14, 2002.
1. Field of the Invention
In general, the present invention relates to turbidity sensors for household appliances, such as dishwashers, washing machines and dryers. In particular, the present invention relates to turbidity sensors wherein the transmission characteristics of single components, for example of a sender and a receiver for optical radiation, are or will be, respectively, matched with respect to each other.
2. Background of the Invention
In household appliances, such as washing machines, dish washers and dryers, it is known to employ turbidity sensors in order to determine the turbidity or dirt level of media used in such household appliances, such as cleaning or washing water and drying air, respectively. For example, it is possible to determine the dirtying of a product to be cleaned on the basis of the turbidity or dirt level of cleaning water or the humidity present in a product to be dried via substances of exhaust air of a dryer or the condition of a fluff sieve of a dryer.
In general, turbidity sensors are comprised of a sender and a receiver for optical radiation that propagates through the used medium. Interactions of the radiation on its propagation path through the medium are used to infer the turbidity or dirt level of the medium.
Turbidity sensors comprises a housing which is arranged in a household appliance such that at least those portions of the housing which comprise the sender and receiver project into the interior of the appliance to direct the radiation through the medium being present at operation of the appliance in its interior.
The transmission characteristic of such turbidity sensors is essential for their operation since it determines accuracy and reliability of turbidity or dirt level detection. For a non damped transmission path between a sender and a receiver of a turbidity sensor, the receiver current generated by the receiver results from a multiplication of the current supplied to the sender for generating the radiation with a constant which is, beside by the parameters given in the following, also affected from those portions of the sensor housing and the medium through which the measuring beam is propagating.
On the sender side, the constant depends from the efficiency of the sender for example the efficiency of a sender diode for optical radiation and the quality of a sender chip used therewith, respectively. Accordingly, on the receiver side, the constant is influenced by the efficiency of the receiver, for example by the quality of a used receiver chip.
To optimize the transmission characteristic of turbidity sensors, i.e. to obtain an output voltage as large as possible or an output current as large as possible on the receiver side for a given voltage or a given current for operation of the sender, usually selected components are used for design of the sender and the receiver. Here, it is known to measure a sender and a receiver and their components, respectively, before design of a turbidity sensor as regards their sending and receiving characteristics, respectively, and, then, to group them accordingly, i.e. to employ senders and receivers matching each other in pairs in turbidity sensors. Due to the required testing of the senders and receivers, this procedure is time consuming and costly and results that, in general, only specific groups of sender and receiver pairings can be used. As an alternative, it is known to supply the sender of a turbidity sensor with power being adjustable over a large range in order to obtain, on the receiver side, signals of desired or required magnitude and quality. This requires a power adjustment on the sender side in dependence of output signals when designing a turbidity sensor. For power adjustment, required components are arranged within the turbidity sensor but are necessary only when designing the turbidity sensor but not for its operation. Beside the required power adjustment in the power supply of the sender, this approach has the drawback that such turbidity sensors comprise components that are not necessary for their actual operation. As an alternative, components required for such a power adjustment are arranged in a unit used for controlling the sensor. In both cases, the sensor is generally operated only at one operating point which is why the components used for power adjustment are not required for actual operation.
Increasingly, such measures are required since the components used in turbidity sensors, in particular senders and receivers, are fabricated with increasingly higher efficiencies and sensitivities (qualities), respectively. In turn, this makes it increasingly difficult to obtain sender-receiver-pairings that exhibit characteristic parameters given for an operation of a household appliance. In other words, the single components for design of a turbidity sensor are increasingly better which is why turbidity sensors exhibit worse measuring characteristics if the sender-receiver-pairings selected for design are not optimally matched with respect to each other.
In general, object of the present invention is to overcome the above named problems of prior art. In particular, the present invention should provide a method for fabrication of turbidity sensors which makes it possible to, on the one hand, use very high quality senders and receivers to compensate their higher absolute values in a simple manner and, on the other hand, to allow for the application of senders and receivers being more imprecise or being selected improperly matched with respect to each other (xe2x80x9cnon-fitting sender/receiverxe2x80x9d). Furthermore, the present invention should provide turbidity sensors having transmission characteristics which compensate the deviation of high quality senders and receivers and which compensate the effects resulting from a combination of imprecise senders and imprecise receivers.
For a solution of the above object, the present invention provides an apparatus for adjusting of output voltages of a turbidity sensor and accordingly constructed turbidity sensors.
In particular, the present invention provides a method for adjusting (e.g. shifting and/or limiting) of output voltages of a turbidity sensor that serve for determination of the turbidity of a medium by means of radiation, at least partially propagating through the medium, which is emitted from a sender and received from a receiver. For characterizing desired output voltages of the receiver in response to radiation received from the sender of a sender current use for operation of the sender, a desired characteristic curve or a desired characteristic curve range is defined. By means of the real components of the turbidity sensor and in particular of the sender and the receiver, a current characteristic curve is determined which indicates the actual dependency of output voltages of the receiver from sender currents. Based on a comparison of the current characteristic curve with the desired characteristic curve or with the desired characteristic curve range, it is determined whether the output voltages of the receiver correspond with the desired output voltages. If this is not the case, the ohmic load of the receiver on its output side is adjusted such that the desired output voltages are obtained.
Preferably, an integrally formed resistance of the turbidity sensor is adjusted, i.e. varied in its magnitude, such that the desired output voltages are obtained. Herewith is contemplated to employ potentiometers or laser trimmed resistances. Further, for adjusting the ohmic load of the receiver on its output side it is possible to determine the transmission characteristic for the radiation (e.g. wave length dependent) and to determine, in dependence therefrom, resistance values which result in the desired output voltages. Preferable, these measurements are performed for a given or known condition of the medium, for example for an unclouded medium or for a medium having a known dirt level.
Further, the present invention provides a method for adjusting output voltages of a turbidity sensor that determines the turbidity of a medium by means of two radiations having different wavelengths. In dependence of the wavelength the receiver of the turbidity sensor receives, its ohmic load on its output side is adjusted such that, irrespective of the wavelength of the received radiation, essentially the same output voltages result.
Further, the present invention provides turbidity sensors having ohmic loads on its output side adjusted according to the previously mentioned methods.